With increasing popularity of mobile voice and data communication, there is an ever increasing demand for high-speed voice and data communication. The licensed spectrum for cellular communication is rapidly being exhausted by a dense and growing subscriber base. This applies in particular to the valuable low-frequency bands with low propagation loss traits.
A significant amount of unlicensed spectrum is available. For illustration, a significant amount of spectrum is globally available in the 5 GHz band. It is desirable to utilize the LTE-unlicensed (LTE-U) spectrum to augment the capacity of licensed frequency carriers. The LTE-U spectrum may be used for carrying data traffic for mobile services. The purpose of LTE-U is to extend LTE cellular communication to unlicensed spectrum.
Carrier aggregation may be performed in which usage of LTE-U spectrum and LTE licensed spectrum are combined. The LTE licensed carrier may be a primary carrier. The LTE-U carriers may be secondary carriers to which data traffic may be offloaded in the downlink or both in the uplink and in the downlink. LTE-U data transmissions may still be controlled by the primary carrier. The secondary cell (Scell) may be logically synchronized to the primary cell (Pcell). LTE-U data transmissions which may include control data and payload data may be started at subframe boundaries which are defined by signalling over primary carriers in the LTE licensed spectrum. The start times of the subframes may be determined by Pcell signalling.
The radio resource in the LTE-U frequency bands which is intended to be used for an LTE-U data transmission must be free at the start time of the subframe at which the LTE-U data transmission is performed. Because the spectrum in the unlicensed frequency bands may also be used by communication techniques different from cellular communication, a communication device, e.g. an eNodeB, will not always be able to transmit when it intends to transmit LTE-U data. The reason for this is that radio resources in the LTE-U spectrum may be co-utilized by Wi-Fi or other non-cellular communication techniques.
It would be desirable to reduce the risk that further communication device start utilizing a radio resource in the LTE-U frequency band in a time gap remaining until the next subframe starts.